1. Field of the Invention
The present invention relates to a projection optical system and an image displaying apparatus.
2. Description of the Related Art
Various techniques for a projection optical system and an image displaying apparatus have been disclosed conventionally.
For example, Japanese Patent Application Publication No. 2001-255462 (JP-A-2001-255462) discloses a projection optical system for light-guiding a light beam from an image displaying panel onto a screen tilted with respect to a reference axis to form an image information on the screen, wherein the projection optical system includes a reflection optical system having plural rotationally asymmetric reflective surfaces having a curvature and reflecting from the plural rotationally asymmetric reflective surfaces a light beam from the image displaying panel to guide light onto the screen and a stop provided between the plural rotationally asymmetric reflective surfaces of the reflection optical system or between the reflection optical system and the image displaying panel, wherein the stop is arranged to be imaged with a negative magnification by an optical member arranged at the screen side of the position of the stop, and the like.
In particular, a projection optical system including six free-form surface mirrors thereby conducting upward reflections from the downside to the upside in order, then forming an intermediate image, then conducting reflection on a concave mirror, and finally conducting projection on a screen by means of oblique incidence is disclosed in FIG. 1 of JP-A-2001-255462.
However, the projection optical system as disclosed in FIG. 1 of JP-A-2001-255462 may have a problem that the height of the apparatus is large because its optical path is folded by the mirrors provided in the vertical directions.
Meanwhile, Japanese Patent Application Publication No. 2006-235516 (JP-A-2006-235516) discloses a projection optical system for enlarging an image on a conjugate plane at the reduction side among a pair of conjugate planes and imaging it on a conjugate plane at the enlargement side, wherein a first imaging system and a second imaging system are arranged in order from the conjugate plane side at the reduction side, wherein the first imaging system includes plural lenses and the second imaging system includes a concave reflecting mirror having an aspherical form, and the optical system is configured to form an intermediate image at a position between the first imaging system and the second imaging system, and the like.
In particular, a projection optical system for forming an intermediate image by a refraction system, then conducting reflection on a concave mirror, and finally conducting projection onto a screen by means of oblique incidence is disclosed in FIG. 1 of JP-2006-235516.
However, the projection optical system as disclosed in FIG. 1 of JP-A-2006-235516 may have a problem that the height of the apparatus is large because the concave mirror has a downward-projecting part although it has no part projecting to the projection display side (above the lens in FIG. 1).
Meanwhile, Japanese Patent Application Publication No. 2004-309765 (JP-A-2004-309765) discloses a reflection and imaging optical system including plural optically reflective surfaces having a curvature, wherein the gap between a pair of optically reflective surfaces through which an entering light beam passes at the time of entering the reflection optical system is identical to the gap between a pair of optically reflective surfaces through which an exiting light beam passes at the time of exiting, and the like.
In particular, an optical system obtaining an intermediate image by a refraction system and a mirror system and conducting enlargement and projection by a concave mirror is disclosed in FIG. 2 of JP-A-2004-309765. More specifically, a projected image is obtained by four optically reflective surfaces (described as rotationally asymmetric surfaces having a curvature in paragraph [0027] of JP-A-2004-309765) whereby the optical path crosses plural times in the inside of the reflection optical system after exiting from the refraction system. Also, an optical system with three surfaces is disclosed in FIG. 5 of JP-A-2004-309765. A reflective surface of the optical system disclosed in each of FIG. 2 and FIG. 5 of JP-A-2004-309765 is an optically reflective surface having a power and has a desired shape based on the design of a projection optical system.
However, the reflection optical system in the optical system as disclosed in each of FIG. 2 and FIG. 5 of JP-A-2004-309765 has a projecting part to a side (upside) opposite to the projection side (downside) so that the apparatus has a projecting bottom and accordingly is large.
Furthermore, because the last surface of the imaging optical system has a power in the optical system as disclosed in each of FIG. 2 and FIG. 5 of JP-A-2004-309765, it has to be arranged in accordance with a desired design value of its position with respect to another optically reflective surface. Herein, it may be possible to control the direction of projection by tilting or moving an optical element but movement of an optical element having a power could cause degradation of the optical performance of projection. That is, it would be very difficult to maintain the projection performance. If a desired angle was intended to set, it would be necessary to re-design the entire optical system. If the optically reflective surface which is the last surface was replaced with a plane mirror that substantially has no power and the shape of another optical element was optimized and designed again in order to ensure the projection performance, the sizes of the optical system such as the height of the entire optical system and the distance between the projection position at the maximum height and the bottom surface of the optical system would not be reduce.
Meanwhile, optical systems installed in a rear projector and including a first optical system and a second optical system are disclosed in FIG. 1 and FIG. 2 of Japanese Patent Application Publication No. 2007-212748 (JP-A-2007-212748).
In particular, a configuration such that light rays reflected from a concave mirror constituting the second optical system are reflected by a folding mirror, traverse a light beam passing between the first optical system and the second optical system, and projected onto a screen is disclosed in FIG. 2 of JP-A-2007-212748. The first optical system and a first reflective surface (for example, a concave mirror in FIG. 2) of the second optical system are arranged to put the normal line of an image plane (for example, a screen) at the center thereof between them. That is, the first optical system is arranged apart from the second optical system while the normal line of an image plane (for example, a screen) at the center thereof is centered.
Such a configuration results in a projection optical system that is larger than the longitudinal dimension of a projected image reflected on a screen. Furthermore, if the positional relationship between the first optical system and the second optical system is not held at a desired positional relationship, the imaging performance may be degraded. That is, there is a problem that it could be an impractical projection system, because a member for holding a concave mirror while its positional relationship with the first optical system is held is large, particularly in the case of its application to a front projector. Moreover, even if reduction of its thickness may be attained in its application to rear projection, there is a problem that the size of projection apparatus is increased, because the first optical system is separated from the second optical system, and accordingly, the space occupied by light traveling in the inside of the optical system is large.
The inventors have conceived of an idea of providing a more compact projection optical system.
The inventors have also conceived of an idea of providing an image displaying apparatus comprising a more compact projection optical system.